1. Field of Invention
The present invention relates to opening of circuit breakers in power distribution networks.
2. Description of the Prior Art
In large alternating current electrical power distribution networks, substation class circuit breakers have long been used both to close and to open or trip an electrical circuit. These types of circuit breakers have been typically rated in terms of their interrupting capacity, typically defined as the maximum instantaneous available fault current which a breaker has been rated to interrupt. Interrupting capacity has typically been measured in kiloamperes (kA).
Typical fault currents have been substantial and existing breaker technology has, so far as is known, relied on bulk and size in a breaker to handle interruption of the high fault current levels expected. For example, in a 69 kilovolt network, a fault current of 40 kiloamperes was a typical rating, while for a 138 kilovolt network a rated fault current was on the order of 80 kiloamperes.
In operation, as the electrical contacts parted, an alternating current electrical arc was established between a stationary contact and a moving contact element located in an insulating medium. As the electrical contacts parted, the arc was formed by ionization of the insulating medium as well as of the materials of the contact elements.
As the contact elements moved further from each other, the arc became elongated and was cooled by the insulating medium. Further provision was also usually made to cool and lengthen the arc, either by magnetically or mechanically applied forces. As the arc length cooled and lengthened, the resistance of the arc path increased. At some point after interruption, the arc path resistance reached a sufficiently high level that the alternating current arc could not re-ignite itself as the alternating current changed polarities. So far as is known, it has been generally accepted practice in circuit breakers to allow an arc to be established and then conditioned, or gradually cooled and lengthened, in order to interrupt a fault.